Conventional methods for manufacturing semiconductor devices commonly include an etching using a reactive ion etching (RIE) method and a film deposition using a plasma chemical vapor deposition (CVD) method. Both the RIE method and plasma CVD method are manufacturing methods which typically include the use of plasma.
However, when conducting a manufacturing method using plasma, ultraviolet radiation is produced. Ultraviolet radiation includes some undesirable side effects to semiconductor fabrication. For example, ultraviolet radiation is known to break a Si—O bond in silicon oxide and a Si—N bond in silicon nitride. As a solution to this problem, flash memories having an oxide-nitride-oxide (ONO) film including a charge storage layer have been provided with an ultraviolet absorbing film which absorbs ultraviolet above the ONO film have been developed. One example of this solution is disclosed in Published Japanese Translation of PCT Application No. JP-T-2007-516598).
FIG. 1A is a top view illustrating the flash memory according to the conventional solution and FIG. 1B is a cross-sectional view taken along the line A-A of FIG. 1A. With reference to FIGS. 1A and 1B, a bit line 12 which also serves as a source and a drain is formed so as to extend in a semiconductor substrate 10. An ONO film 20 composed of a tunnel insulating film 14, a charge storage layer 16 and a top insulating film 18 is formed above the semiconductor substrate 10. ,A a word line 22 which also serves as a gate is formed on the ONO film 20, so as to extend crossing the bit line 12. An interlayer insulating film 24, an ultraviolet absorbing film 26, an anti-reflection film 28 and a cap layer 30 are formed in sequence on the word line 22. A contact hole is formed which runs through the cap layer 30, the anti-reflection film 28, the ultraviolet absorbing film 26, the interlayer insulating film 24 and the ONO film 20 and, a plug metal 32 is formed so as to be embedded in the contact hole. With such a configuration, as the ultraviolet absorbing film 26 absorbs the ultraviolet produced when forming the contact hole by an RIE method, the amount of ultraviolet that reaches the ONO film 20 is suppressed and the damage the ONO film 20 receives can be prevented.
For the ultraviolet absorbing film 26, a silicon-rich oxide film can be used. However, in the silicon-rich oxide film, when an extinction coefficient for ultraviolet (hereinafter, the extinction coefficient) is varied, an etching rate also varied. As the extinction coefficient is varied by the ratio of silicon included in the silicon-rich oxide film, a variation in extinction coefficient will result according to the conditions between devices or inside a chamber of even the same device used for depositing a silicon-rich oxide film. Therefore, an ultraviolet absorbing film 26 having various etching rates may be formed.
FIGS. 2A and 2B are cross-sectional views taken along the line equivalent to the line A-A in FIG. 1A illustrating the formation of a contact hole in a flash memory according to the conventional solution in which the ultraviolet absorbing film 26 having a desired extinction coefficient is formed.
With reference to FIGS. 2A and 2B, forming the contact hole is typically carried out in two separate etching processes. More specifically, a first etching process and a second etching process are carried out under different etching conditions. This is because, as the ultraviolet absorbing film 26 is a silicon-rich oxide film, the etching rate of the ultraviolet absorbing film 26 is slower than that of the interlayer insulating film 24 and such. In both the first etching process and the second etching process, etching is carried out with a fixed etching time.
With reference to FIG. 2A, in the first etching process, a first opening 36 is formed with a photoresist 34 formed on the cap layer 30 as a mask, by etching from the cap layer 30 down to a part of the interlayer insulating film 24. Since the extinction coefficient of the ultraviolet absorbing film 26 is of the desired value, the etching rate of the ultraviolet absorbing film 26 also becomes a desired rate. Therefore, the first opening 36 can be formed such that a bottom surface of the first opening 36 comes to a desired depth of approximately 100 nm from the border between the interlayer insulating film 24 and the ultraviolet absorbing film 26.
With reference to FIG. 2B, in the second etching process, a second opening 38 is formed which runs through the cap layer 30, the anti-reflection film 28, the ultraviolet absorbing film 26, the interlayer insulating film 24 and the ONO film 20 by etching the interlayer insulating film 24 and the ONO film 20 below the first opening 36. Consequently, a contact hole made of the second opening 38 can be formed for a desired shape.
FIGS. 3A and 3B are cross-sectional views taken along the line equivalent to the line A-A in FIG. 1A illustrating the forming of a contact hole in an alternate example of a conventional flash memory having an ultraviolet absorbing film 26 of an extinction coefficient larger than that of the desired value
With reference to FIG. 3A, when the extinction coefficient of the ultraviolet absorbing film 26 is large, as the ratio of silicon included in the ultraviolet absorbing film 26 is large, the etching rate of the ultraviolet absorbing film 26 becomes small. As the etching time is fixed for forming the first opening 36, the first opening 36 may result in being formed up to halfway through the ultraviolet absorbing film 26. Under this condition, when forming the second opening 38 is carried out, as shown in FIG. 3B, the second opening 38 results in a tapered shape of which width gradually narrows towards a bottom surface.
On the other hand, when the extinction coefficient of the ultraviolet absorbing film 26 is smaller than the desired value, as the ratio of silicon included in the ultraviolet absorbing film 26 is small, the etching rate of the ultraviolet absorbing film 26 becomes large. Therefore, the depth of the first opening 36 becomes deeper than as shown in FIG. 2A. Under this condition, when forming the second opening 38 is carried out, as the etching time is fixed for forming the second opening 38, the bit line 12 formed below the second opening 38 is being etched for a longer time and the bit line 12 may get damaged.
As described above, the extinction coefficient of the ultraviolet absorbing film 26 may result in various values, therefore, the etching rate of the ultraviolet absorbing film 26 may be performed according to various rates. As shown in FIGS. 2A to 3B, the shape of the second opening 38 depends on the depth of the first opening 36, more specifically, depends on the etching rate of the ultraviolet absorbing film 26. Therefore, when the etching rate of the ultraviolet absorbing film 26 is in various rates, stably forming the second opening 38 of the desired shape is difficult. More specifically, stably forming the contact hole of the desired shape is difficult. Consequently, an electrical resistance of a contact plug formed so as to be embedded in the contact hole results in various values, and a desired electrical resistance is difficult to obtain.
FIG. 4 is a flowchart illustrating a method for manufacturing the flash memory according to a proposed solution to the above mentioned problem. FIG. 4 provides a a flash memory with a bit line 12 formed so as to extend in the semiconductor substrate 10. Above the semiconductor substrate 10, the ONO film 20 is formed by forming the tunnel insulating film 14, the charge storage layer 16 and the top insulating film 18 in sequence. On the ONO film 20, a word line 22 is formed (step S10) so as to extend crossing the bit line 12. On the word line 22, the interlayer insulating film 24 is formed (step S12).
Subsequently, in a separate batch from the manufacturing of the flash memory, the ultraviolet absorbing film 26 is formed on a dummy substrate. This may also serve as a confirmation of operations conducted every day as a daily checkup on a device used for forming the ultraviolet absorbing film 26. The extinction coefficient of the ultraviolet absorbing film 26 formed on the dummy substrate is measured. Whether the extinction coefficient is within the desired range or not is confirmed (step S14). When the extinction coefficient is within a desired range, a step S18 is proceeded to. When the extinction coefficient is not within the desired range, a film deposition condition of the ultraviolet absorbing film 26 is altered (step S16). Under the altered film deposition condition, the ultraviolet absorbing film 26 is formed on the dummy substrate and the extinction coefficient of the ultraviolet absorbing film 26 is measured again. Whether the extinction coefficient is within the desired range or not is confirmed (step S14). Until the extinction coefficient falls within the desired range, the step S16 and the step S14 are repeated.
Furthermore, the process of forming the ultraviolet absorbing film 26 on the dummy substrate and confirming whether the extinction coefficient of the ultraviolet absorbing film 26 is within the desired range or not (step S14) is not necessarily carried out after the interlayer insulating film 24 is formed (step S12), hence may be carried out before forming the interlayer insulating film 24 (step S12) or before forming the bit line 12 and such (step S10), more specifically, at any time before forming the ultraviolet absorbing film 26 (step S18) in the manufacturing of the flash memory of the third comparative example.
On the interlayer insulating film 24, the ultraviolet absorbing film 26 is formed (step S18). After the step S14, the extinction coefficient of the ultraviolet absorbing film 26 formed on the interlayer insulating film 24 is of the desired value. More specifically, the etching rate of the ultraviolet absorbing film 26 becomes the desired rate. Next, on the ultraviolet absorbing film 26, the anti-reflection film 28 and the cap layer 30 are formed in sequence (step S20).
The first opening 36 is formed (step S22) with the photoresist 34 formed on the cap layer 30 as a mask, by etching from the cap layer 30 down to a part of the interlayer insulating film 24 under a predefined etching condition. As the etching rate of the ultraviolet absorbing film 26 is of the desired rate, even when the etching time for forming the first opening 36 is fixed, the first opening 36 of the desired depth as shown in FIG. 2A can be formed.
By etching the interlayer insulating film 24 and the ONO film 20 below the first opening 36 under a predefined etching condition, the second opening 38 that runs through the cap layer 30, the anti-reflection film 28, the ultraviolet absorbing film 26, the interlayer insulating film 24 and the ONO film 20 and exposes the bit line 12 is formed (step S24). Consequently, the contact hole made of the second opening 38 is formed. The plug metal 32 is formed (step S26) by embedding a metal in the second opening 38 (the contact hole).
According to the method for manufacturing the flash memory of the third comparative example, the ultraviolet absorbing film 26 having a desired extinction coefficient can be formed. More specifically, the ultraviolet absorbing film 26 having a desired etching rate can be formed. Therefore, as the shape of the second opening 38 can be prevented from being in a tapered shape or the bit line 12 below the second opening 38 can be prevented from being damaged, the second opening 38 of a desired shape can be stably formed.
However, in the method for manufacturing the flash memory as shown in FIG. 4, until the extinction coefficient of the ultraviolet absorbing film 26 reaches the desired value, the step S14 and the step S16 must repeatedly be carried out. Accordingly, this process may become inefficient and suboptimal as a result of the repetition.